1. The Field of the Invention
The present invention relates to vertical cavity surface emitting lasers. More particularly, the present invention relates to single mode vertical cavity surface emitting lasers that use photonic crystals with a central defect.
2. Background and Relevant Art
Vertical cavity surface emitting lasers (VCSELs) are an example of semiconductor lasers used in fiber optical systems and have several advantages over other types of semiconductor lasers. VCSELs can be manufactured in large quantities due to their relatively small size and can often be tested on a single wafer. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well with optical fibers.
VCSELs are typically made from both GaAs semiconductor materials and InP semiconductor materials, but GaAs semiconductor materials make better multi-layer mirror systems than InP semiconductor materials. A high reflectivity mirror system is needed in a VCSEL because the light resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has little chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity is currently achieved using Distributed Bragg Reflector (DBR) layers.
One problem associated with VCSELs is related to the wavelength of the light that is generated. Current VCSELs typically generate light that has a wavelength of approximately 0.85 microns. This wavelength is primarily useful in very short distance fiber optic communications but is typically inadequate for longer distance fiber optic networks such as telecommunication networks. Attempts to develop and fabricate VCSELs that operate at longer wavelengths (1.3 microns and 1.55 microns, for example) and at higher power have proven to be very difficult. This difficulty is related to the fact that InP semiconductor materials permit the growth of a suitable active region for generating longer wavelengths, but the InP DBR layers are not effective. When GaAs semiconductor materials are used, the growth of the DBR layers is straightforward, but the active region is unsuitable. In addition, attempts to increase the power produced by VCSELs results in multimode emission. The difficulty in fabricating and designing the multiple DBR layers and the need for the lattice structures of the various layers in the VCSEL to match are additional reasons that impede the successful creation of a high power single mode VCSELs. Typically, attempts to create such VCSELs have resulted in VCSELs that produce insufficient power, are unreliable, or generate multiple modes.
These and other limitations are addressed by the present invention, which relates to a single mode vertical cavity surface emitting lasers using photonic crystals with a central defect. More specifically, a single mode is achieved by forming a mirror layer of a VCSEL using photonic crystals or a combination of photonic crystals and Distributed Bragg Reflector (DBR) layers. The photonic crystal also includes a central defect to facilitate propagation of the emitted mode of laser light. Power in the single mode can be increased without the emission of addition modes.
A photonic crystal or layer is a material, such as a semiconductor material or a dielectric material, in which cavities or holes are formed. The cavities or holes formed in the photonic crystal usually have a periodic structure. The periodic cavity structure of a photonic crystal is not limited by the atomic lattice structure of the material and can be formed as required using various lattice configurations and cavity shapes. Because the photonic crystals used in the present invention are planar in nature, the periodic cavity structure is usually two dimensional, although a three dimensional photonic crystal is contemplated by the present invention. A central defect is created when cavities or holes are not formed in a portion of the photonic crystal. Typically, the central defect corresponds to an aperture of the VCSEL where laser light is emitted.
In one embodiment of the present invention, a photonic crystal with a central defect is formed on the upper DBR layers of a VCSE. Alternatively, periodic cavity structure is formed in the DBR layers, thereby making the DBR layers photonic. The reflectivity of the photonic crystal is dependent on the wavelength of the light and on the angle of incidence. The photonic crystal provides the necessary reflectivity for a single mode such that a single mode is reflected through the active region, which results in stimulated emission of photons at the corresponding wavelength of the incident photon. The photonic crystal does not provide sufficient reflectivity for other modes and as a result, those modes do not have appreciable gain.
The particular mode or wavelength emitted by a VCSEL can be changed by varying or altering attributes or characteristics of the photonic crystal. Exemplary attribute changes include, but are not limited to, changing the cavity structure to another lattice configuration, changing the dimensions of the central defect, altering the shape of the individual cavities, adding another photonic crystal or layer to the VCSEL, and the like or any combination thereof. With photonic crystals as mirrors, longer wavelengths can be generated by the VCSEL. The VCSEL can also be configured to emit a particular wavelength by controlling the refractive index of the photonic crystal by filling the cavities with another material. Additional layers of photonic crystals may extend the band of wavelengths for which high reflectivity is achieved.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.